The circuits of Chapter 28, with a steady current in one direction, are called DC circuits—direct current. A circuit with an oscillating emf is called an AC circuit, for alternating current. The wires that transport electricity across the country—the grid—use alternating current.

《 LOOKING BACK Chapter 28 Circuits

Resistors in an AC circuit act as they do in a DC circuit. But you’ll learn that capacitors and inductors are more useful in AC circuits than in DC circuits.

• The voltage across and the current through a capacitor or inductor are 90° out of phase. One is peaking when the other is zero, and vice versa.

• The peak voltage $V$ and peak current $I$ have an Ohm’s-law–like relationship $V=IX$, where $X$, which depends on frequency, is called the reactance.

• Unlike resistors, capacitors and inductors do not dissipate energy.

《 LOOKING BACK Section 26.5 Capacitors

《 LOOKING BACK Section 30.8 Inductors

AC voltages oscillate sinusoidally, so the mathematics of AC circuits is that of SHM. You’ll learn a new way to represent oscillating quantities—as a rotating vector called a phasor. The instantaneous value of a phasor quantity is its horizontal projection.

《 LOOKING BACK Chapter 15 Simple harmonic motion and resonance

A circuit with a resistor, inductor, and capacitor in series is called an $RLC$ circuit. An $RLC$ circuit has a resonance—a large current over a narrow range of frequencies—that allows it to be tuned to a specific frequency. As a result, $RLC$ circuits are very important in communications.

AC circuits are the backbone of our technological society. Generators automatically produce an oscillating emf, AC power is easily transported over large distances, and transformers allow engineers to shift the AC voltage up or down. The circuits of radio, television, and cell phones are also AC circuits because they work with oscillating voltages and currents—at much higher frequencies than the grid, but the physical principles are the same.